Zinc-cadmium antimonide single crystal anisotropic thermoelement

ABSTRACT

An anisotropic thermoelement is manufactured from one single crystal possessing anisotropic thermal electromotive force in at least two mutually perpendicular directions. A temperature gradient applied to said single crystal at some angle relative the direction of anisotropy of the thermal electromotive force causes the emergence of electromotive force in a direction perpendicular to that of the temperature gradient applied. Used as the single crystal is ZnxCdl xSb solid solution, where x assumes a value greater than zero but less than unity; in a limiting case has a value of 0.99 recurring.

Unite Sates atent Pilat et a1.

1 1 Jan. 15, 1974 ZINC-CADMIUM ANTIMONTDE SINGLE CRYSTAL ANISOTROPIC THERMOELEMENT Inventors: Izrail Moiseevich Pilat, ulitsa Zankovetskaya, 15, kv. 29; Anatoly Grigorievich Samoilovich, ulitsa Universitetskaya, l3, kv. 4; Kornei Denisovich Soliichuk, ulitsa Lenina, 145, kv. 28, all of Chernovtsy; Eduard Vaganovich Osipov, ulitsa Lasovskogo, 3a, kv. 32, Kiev, all of U.S.S.R.

Filed: Sept. 27, 1971 Appl. No.: 184,275

Related U.S. Application Data Continuation of Ser, No. 226, Jan. 2,

abandoned.

U.S. Cl 136/205, 136/236, 136/240 Int. Cl H0lv 1/20 Field of Search 136/240, 205, 213,

References Cited UNITED STATES PATENTS 9/1970 Samoilovich et al. 136/213 3,211,517 10/1965 Castellion 136/240 X 3,454,370 7/1969 Castellion 136/240 X 3,021,378 2/1962 Justi et al 252/623 Primary Examiner-Leland A. Sebastian Assistant Examiner-E. A. Miller Attorneyl-lolman, Glascock, Downing & Seebold [57] ABSTRACT 5 Claims, 4 Drawing Figures ZINC-CADMIUM ANTIMONIDE SINGLE CRYSTAL ANISOTROPIC TI-IERMOELEMENT This is a continuation of US. Pat. application Ser. No. 226 filed Jan. 2, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to devices converting thermal energy into an electric potential, and, more particularly, it relates to anisotropic thermoelements.

2. Description of Prior Art In the hitherto known thermoelements (thermocouples) the presence of two elements of different substances and temperature difference constitute necessary pre-requisites for the emergence of a thermal electromotive force.

Such thermoelements, manufactured both of metals and semiconductors, however suffer from a number of inherent disadvantages. Among the disadvantages are low electromotive force, interdiffusion from one element into the other, counter thermal electromotive force due to mixed conductivity, transition resistances at places of the element contacts (thermoelement commutation effect), low reliability and short service life due to the presence of layers between the thermoelement members and diffusion between elements.

There are likewise known thermoelements consisting of a single crystal of CdSb compound (Patent Applications filed by the present Applicants in the U.S., FRG and Japan), wherein some of the above-mentioned difficulties can be overcome. However, such thermoelements feature a relatively high internal resistance, which reduces the power yielded by the thermoelement. Besides, when using said material, the anisotropy obtained of thermal electromotive force above room temperature does not exceed Au 150 mu v/deg.

SUMMARY OF THE INVENTION It is an object of the invention to eliminate the aforementioned disadvantages.

The main object of the present invention is to provide a small-sized anisotropic thermoelement featuring high sensitivity, increased reliability and extended service life, by using a single crystal of special composition.

In the accomplishment of said and other objects of the present invention, an anisotropic thermoelement manufactured from one single crystal possessing anisotropic thermal electromotive force in at least two mutually perpendicular directions while the temperature gradient applied to said single crystal at some angle relative the direction of anisotrophy of the thermal electromotive force causes the emergence of electromotive force in a direction perpendicular to that of the temperature gradient applied; according to the invention, the single crystal used in Zn,Cd, Sb solid solution, where is in the range between and 0.9 recurring.

The anisotropic thermoelement proposed herein provides for the increse of electromotive force produced by at least several times as compared with the known thermocouples. As compared to the known anisotropic thermoelement manufactured from CdSb the proposed thermoelement provides for electromotive force exceeding by 50 per cent of that of conventional thermoelements of the same size and for the same temperature differences; the maximum yielded power of the proposed thermoelement exceeds by over one order of that yielded by a conventional CdSb thermoelement of the same size.

The proposed anisotropic thermoelement is simple in design, reliable in operation, has a prolonged service life, does not age, and is small in size.

Such anisotropic thermoelements have found wide application in measurement technology, automatics and instrument making. They can be used in designing high-sensitivity instruments for registering various irradiations, measuring temperature gradients and heat fluxes, as well as thermal converters in electric measuring instruments etc.

BRIEF DESCRIPTION OF THE DRAWINGS The prsent invention will become apparent upon considering a detailed description thereof taken in conjunction with the accompanying drawings, wherein:

FIG. illustrates a single crystal in section, showing X, Y, and Z-axes, which represent respectively direction of crystallographic axes direction of electromotive force obtained and, direction of the applied temperature gradient FIG. 2 shows temperature dependence curves A0: and A01 in the planes (I00) and (001) of a single crystal of p-type Zn Cd Sb solid solution within the 250400 K. range; A01 =a a Acr =oz a FIG. 3 shows the thermal electromotive force of a thermoelement, manufactured from a single crystal of p-type Zn Cd Sb solid solution, as a function of the applied temperature difference; and

FIG. 4 shows load characteristics at different temperatures for a thermoelement manufactured from a single crystal of p-type Zn CdMSb solid solution.

A single crystal possessing anisotropic thermal electromotive force is considered hereinbelow. For the sake of simplicity the case is considered when the thermal emf tensor has at least two different components, that is, the thermal electromotive force is different in two mutually perpendicular directions. The thermal emf anisotropy is usually connected with crystallographic directions. Let a tensor component denote thermal electromotive force along the crystallographic axis tensor component a denote thermal electromotive force of the single crystal l occurring along the crystallographic axis (010) (FIG. 1) in the presence of temperature, denote the thermal electromotive force gradient in the same direction, and (1 respectively in the direction (00l A case is considered when the temperature gradient in the single crystal is directed arbitrarily with respect to the crystallographic axes and is in the plane of the latter axes. Angle d) is equal to the angle between Cartesian axis Y and crystallographic axis (001 The temperature gradient is directd along Z-axis. It can be shown that, due to anisotropy of the thermal electromotive force, along the Y-axis there emerges electromotive force E which can be determined from the formula:

E, sin2 (a #1 (dt/dz) a where a is the size of the crystal along Y-axis (length),

T is temperature. .As seen from the formula (I the maximum electromotive can be obtained at (b 45. With the linear temperature distribution over the crystal along Z-axis and at (1: 45, the formula (1 takes the following form:

where T and T are temperatures at opposite facets of the crystal, and

b is the size of the crystal on Z-axis (thickness).

It is seen from the formula (2) that the value of the emerging thermal electromotive force depends not only a upon the properties of the material and temperature difference, as is the case with conventional thermoelements, but is proportional to the crystal length and inversely proportional to the crystal thickness b. Therefore, the required value of electromotive force can be obtained (other things being equal) by simply selecting the element dimensions. This provides for the possibility of obtaining greater values of electromotive force.

According to the above-described principle, several thermoelements were manufactured from single crystals of Zn,Cd, ,,Sb (where x O0.1) solid solutions grown by the horizontal zonal recrystallization technique with seeding. On the grown single crystals of Zn Cd Sb were carried out measurements of anisotropy of electric, the thermoelecric and galvanomagnetic properties. These investigations have shown that a single crystal of Zn Cd Sb features great advantages shown in FIG. 2 for a given single crystal are temperature dependences of anisotropy of thermal electromotive force, A111 and A01 in the plane (100) and (001), where A01 a 0: and Aot or a Substantial anisotropy of thermal electromotive force is observed at temperatures above 250 K, and, at 400 K., the anisotropy of thermal electromotive force reaches the maximum value (a -ar 245 mu v/deg.).

Of all the investigated substances featuring anisotropic thermal electromotive force, the proposed material (single crystal of Zn Cd Sb solid solution) for manufacturing anisotropic thermoelements possesses the greatest anisotropy of thermal electromotive force.

Single crystals of Zn Cd, Sb solid solutions, grown with a specific crystallographic orientation, were used to manufacture a number of anisotropic thermoelements cut from the single crystals in accordance with FIG. 1. The elements were mounted on a copper unit used as a heat-sink for the removal of heat. The anisotropic thermoelements were tested under the following conditions: one facet of the thermoelement was maintained at 296 K., while the temperature of the second facet was set by a heater and varied from room temperature to 450 K. The temperatures were measured by copper-constantan thermocouples. In particular, FIG. 3 shows the electromotive force of a thermoelement (a 0.82 cm, b 0.12 cm, c 0.1 cm) manufactured from a single crystal of Zn Cd Sb solid solution as a function of the applied temperature difference. The obtained linear dependence of electromotive force at the thermoelement output upon the applied temperature difference is of particular importance in obtaining linear scale of measuring instruments manufactured on the basis of said thermoelements. The maximum thermal electromotive force is attained at the temperature difference T T, 154 K. and is equal to 108 mv.

The basic distinguishing feature of the present thermoelements consists in that, owing to the employment of single crystals of Zn Cd Sb solid solution featuring highly anisotropic thermal electromotive force, properties electromotive force values exceeding by about 50 per cent of that of thermoelements of the same size and at the same temperature differences applied but manufactured on the basic CdSb single crystal, could be obtained.

Besides, the present results are not optimal and can be improved by increasing the ratio a/b of the anisotropic thermoelement and the temperature gradient applied.

In order to determine the internal resistance of the thermoelement, load characteristics (FIG. 4) were plotted at different temperatures. As seen from the peaks of the load characteristics, the maximum internal resistance of 86 ohm corresponds to 313 K. and the minimum internal resistance of 41 ohm corresponds to 373 K. The maximum yielded power is 71.2 mu w which exceeds by over one order that yielded by a similar but conventional thermoelement of the same size manufactured of CdSb; for a thermoelement manufactured of CdSb the maximum anisotropy of thermal electromotive force is 150 v/deg as compared to 245 mu v/deg at similar temperatures for a thermoelement Of zno cdn gsb.

It should be noted that, by appropriately alloying the initial material and varying the size of the thermoelement depending upon its particular application, one can attain even better parameters. The abovedescribed thermoelements can be modified to further utilize the possibilities of obtaining greater electromotive forces. In particular, the temperature difference set can be increased, the thermoelement thickness reduced, and the overall length extended.

We claim:

1. An antisotropic thermoelement formed of a single crystal with anisotropic thermal emf characteristics, for producing an emf responsive to and determined by a temperature differential applied to the crystal, said crystal having first, second and third substantially perpendicular crystallographic axes and corresponding first, second and third values of thermal emfs that can be produced along said respective axes, the crystal having a thickness in a first direction (Z) extending in a plane defined by those two of said axes (001 and 010) between which the difference of thermal emfs is the maximum, said direction being inclined at a predetermined angle to one of the axes defining said plane, means for applying a predetermined temperature gradient to said crystal in said first direction, and means for tapping a generated output emf in a second direction (Y) defining a length of the crystal, said second direction extending in said plane defined by said axes (001 and 010) between which the difference of thermal emfs is the maximum and being perpendicular to the first direction, and wherein said predetermined angle is greater than zero but less than 90, and wherein said single crystal is composed of a solid solution of Zn, Cd, ,Sb, where 0 x 1.

2. An anisotropic thermoelement as claimed in claim 1 where said predetermined angle is 45.

3. An anisotropic thermoelement as claimed in claim 1 where said length of the crystal is greater than its said thickness.

temperature gradient across the crystal in a first direction which makes a predetermined angle of between 0 and with either one of the two selected axes, said first direction lying in a plane defined by said two selected axes; determining a second direction which also lies in the plane defined by said two selected axes and which is perpendicular to said first direction; and tapping an output emf across faces of the crystal in said second direction, said output emf being proportional to and representative of the applied thermal gradient. 

2. An anisotropic thermoelement as claimed in claim 1 where said predetermined angle is 45*.
 3. An anisotropic thermoelement as claimed in claim 1 where said length of the crystal is greater than its said thickness.
 4. An anisotropic thermoelement as claimed in claim 1 where the value of x is 0.1.
 5. A method of producing an output emf responsive to and determined by a temperature differential applied to an anisotropic thermoelement, using a single crystal of a solid solution of Znx Cd1 xSb where 0< x<1, which comprises: determining the values of thermal emfs that can be produced along first, second and third mutually perpendicular crystallographic axes of the crystal; selecting two of saiD crystallographic axes between which the difference of the values of thermal emfs is the maximum; applying a predetermined temperature gradient across the crystal in a first direction which makes a predetermined angle of between 0* and 90* with either one of the two selected axes, said first direction lying in a plane defined by said two selected axes; determining a second direction which also lies in the plane defined by said two selected axes and which is perpendicular to said first direction; and tapping an output emf across faces of the crystal in said second direction, said output emf being proportional to and representative of the applied thermal gradient. 